A characteristic of the immune system is the specific recognition of antigens. This includes the ability to discriminate between self and non-self antigens and a memory-like potential that enables a fast and specific reaction to previously encountered antigens. The vertebrate immune system reacts to foreign antigens with a cascade of molecular and cellular events that ultimately results in the humoral and cell-mediated immune response.
The major pathway of the immune defense involving antigen-specific recognition commences with the trapping of the antigen by antigen presenting cells (APCs), such as dendritic cells or macrophages, and the subsequent migration of these cells to lymphoid organs (e.g., thymus). There, the APCs present antigen to subclasses of T cells classified as mature T helper cells. Upon specific recognition of the presented antigen, the mature T helper cells can be triggered to become activated T helper cells. The activated T helper cells regulate both the humoral immune response by inducing the differentiation of mature B cells to antibody producing plasma cells and the cell-mediated immune response by activation of mature cytotoxic T cells.
The thymus has been shown to be an obligatory factor in T cell differentiation of hematopoietic cells. Based upon the murine model, it is believed that the presence of a three dimensional organ is required, as in vitro models that do not include the thymus and a three dimensional structure fail to support T cell lymphopoiesis (Owen J J, et al., Br Med Bull., 1989, 45:350–360). The process of differentiation, however, appears to begin prior to progenitor cells contacting the thymus.
Primitive hematopoictic progenitors in the fetal liver or bone marrow give rise to lineage committed cells, including progenitors committed to the T lymphoid lineage. These most immature cells are identified by the surface expression of CD34. T cell lineage committed cells express CD34, but no discrete expression of other epitopes found only on T cell progenitors has been described. Further, T lymphocyte differentiation normally occurs via a series of discrete developmental stages. Primitive progenitor cells which do not express lymphocyte specific cell surface markers (CD34+ CD3− CD4− CD8−) migrate to the thymus where they acquire, through a series of maturational events, the phenotype CD34− CD3− CD4+ CD8−. These cells then mature into double positive CD4+ CD8+ cells, most of which are CD3+, although CD3 expression is not universally detectable. These cells further undergo both positive and negative selection, and mature to develop into single positive T cells (CD4+ CD8− or CD4− CD8+). These cells ultimately migrate into the peripheral circulation as naive T cells.
T cell disorders and diseases represent major health problems. Recent progress has been made using gene therapy to treat conditions involving T lymphocytes, including AIDS. This has fostered increased interest in the development of laboratory techniques that allow in vitro evaluations of potential genetic therapies for these conditions.
The understanding of T cell differentiation has been hampered by the limited availability of technologies which permit in vitro T cell differentiation. To date, T cell differentiation studies have been largely confined to the SCID-hu mouse in vivo model. In vitro technologies have been based on thymic explant studies and primate thymic monolayers. In a recent advance, primate thymic stroma cultures have been shown to provide an expedient, although inefficient, system for examining T cell development, enabling in vitro T cell differentiation in a reproducible manner. However, the purity and number of T cells generated this way, as well as the relatively short half-life of the cultures, generally results in limited applicability to more advanced studies of T cell differentiation and function.